Electric resistance heaters



Nov. 7, 1957 MH WS 3,351,742

ELECTRIC RES I STANCE HEATERS FiledDec 2, 1964 4 Sheets-Sheet l INVEN DARREL M. HAR

ATTORNEY Nov. 7, 1967 D. M. HARRIS ,3

ELECTRIC RESISTANCE HEATERS Filed 0 2, 1964' 4 Sheets-Sheet 2 TEMPERATURE CHACTE RlSTlCS QF UNCOMPENSATED HEATER TEMPERATURE VS BRIDGE LENGTH TEMPERATURE IIIO i I I 70 I I I I I 1 I I l. -2 4 e-"sf I'O l2 I4 16 IB' 20.22'24 INCHES ALONG BRIDGE LENGTH 5 v INVENTOR DARREL' M. HARRIS ATTO R N EY Nov; 7, 1967 r o. M. HARRIS I 3,351,742 ELECTRIC RESISTANCE HEATERS Filed Dec. 2 1964 4 Sheets-Sheeta TEMPERATURE CHACTERISTICS or COMPENSATED BRIDGE TEMPERATURE vs. BRIDGE LENGTH IIQO- (a 33VOLTS 465'AMPS nao n70 (b) 30 VOLTS 455 AMPS n50 TEMPERATURE (c) 27 VOLTS 440 AMPS nook IOQO- INCHES ALONG BRIDGE LENGTH INVENTOR DARREL M. HARRIS ATTORNEY Nov. 7, 1967 D. M. HARRIS 3,351,742

' I ELECTRIC RESISTANCE HEATERS 1 Filed Dec. 2, 1964 TEMPERATURE CHARACTERISTICSOF COMPENSATED BRIDGE TEMRERATURE VS. BRIDGE LENGTH LEFT SIDE OF BRIDGE TEMPERATURE v ,C 70 I I I 50 I 'I I I I I I I I '2 4 6 8, I0 12 14 R3 I8 20 22 INCHES ALONG BRIDGE LE GTH F IG 7 TEMPERATURE CHARACTERISTICS F coMP NsATED BRIDGE I E TEMPERATURE vs BRIDGE LENGTH TEMPERATURE Y .RIGHT SIDE OF BRIDGE 5 y v 40, I

I I.I I I I I I I 2 4 6 e |o l2 'l4 l6 I8 20 22 INcH s ALONG BRI GE LENGTH ATTORNEY 4 Sheets-Sheet 4 United StatesPatentO ice a 3,351,742 ELECTRIC RESISTANCE HEATERS Darrel M. Harris, Kirkwood, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed Dec. 2, 1964, Ser. No. 415,363

5 Claims. (Cl. 219552) ABSTRACT OF THE DISCLOSURE This invention relates in general to certain new and useful improvements in heating devices and more particularly to an improved method for maintaining temperature uniformity across the length of electric resistance heaters.

In recent years, semiconductor devices such as silicon controlled rectifiers have found widespread use in the electronics industry. These solid state rectifiers, such as the silicon controlled rectifiers are often formed by de,

positing an epitaxial silicon film on'wafers formed of generally high purity silicon. The silicon wafers are normally placed upon a graphite heating element which forms part of an epitaxial silicon furnace and are heated to a temperature where free silicon is deposited on the wafer and becomes bonded to the, surface of the water, These wafers are then further processedby conventional methsuch as silicon controlled rectifiers.

In the recent years, it has become a common practice to employ resistance heating elements formed of graphite in these epitaxial silicon furnaces. These heating elementsare generally U-shaped in horizontal cross section and consists of a pair of legs which are connected by a bight portion. The legs are generally provided with terminal connectors at their free ends or the ends remote from the bight portion for ultimate connection to a suitable source of electrical current. A suitable amount of electrical current is then passed through the heating element to heat the element to the desired reaction temperature. However, by reason of the fact that graphite is suitable for use as an electrical heating element, it inherently includes a certain amount of internal resistance which interferes with the passage of electrical current. Therefore, as the electrical current traverses the length of each of the legs in the heating element, a slight voltage drop is developed across the entire length of the heating element and the temperature at the ends of the legs which are connected to the bight portion suffer a ods and used in the manufacture of solid state devices temperature drop. The terminal connectors are norm-ally secured in clamps which are Water cooled. Consequently, there is a rather large heat loss near the terminal ends of the legs which are secured to the clamps. As a result thereof, the ends of the legs are cooler than the center portion-thereof with the result of a non-linear temperature distribution across the legs forming part of the graphite heater. Moreover, there are local hot spots or heat concentrations along portions of the 'legs' forming part of the heater. 7 a

As a result of this occurrence, graphite heating elements have been seriously limited in their effective length,

3,351,742 Patented Nov. 7, 1967 manufacture.

It is. a further object of the present invention to provide a heating element of the type stated which can be conveniently and inexpensively modified for accommodation to the conditions of a furnace.

It is also an object of the present invention to provide a method of selectively altering the cross sectional area of selected portions of a heating element to maintain desired resistance characteristics throughout its effective length for attaining uniformity of temperature.

With the above and other objects in view, my inventi0n'resides in the novel features of form, construction, arrangement, and combination of parts presently described andpointed out.

In the accompanying drawings (4 sheets): 7

FIGURE 1 is a perspective view of a heater construct ed in accordance with and embodying the present invention; I v 1 7 FIGURE 2 is a vertical sectional view taken along lines 22 of the heater of FIGURE 1;

' FIGURE 3 is a perspective view of a modified form of heater constructed in accordance with and embodying the present invention; i g 1 FIGURE 4 is a vertical sectional view taken along line 44 of FIGURE 3; y

FIGURE 5 is a graph of the temperature characteristic of an uncompensated bridge showing temperature Versus bridge length;

FIGURE 6 is a graph of the temperature characteristic of abridge compensated by selectively drilling apertures in accordance with the method of the present invention, showing temperature versus bridge length for various power settings;

FIGURE 7 is a graph of the temperature characteristic of a modified form of compensated bridge constructed in accordance with the present invention and showing temperature versus bridge length for one leg of thebridge;

and

FIGURES is a graph of the temperature characteristic of thebridge of FIGURE 7 showing temperature versus bridge length for the other leg of the bridge.

Generally speaking, the present invention is concerned with graphite heating elements, and .a method of altering artiwas completely devoid of an explanation. for nonuniform thicknesses of epitaxial silicon layers. It has been'found in connection with the present invention, that film quality can be maintained with a high degree of control by maintaining a substantially uniform temperaparticular epitaxial silicon 9 a ture throughout the entire length of the graphite heater. Moreover, uniformity of temperature in the epitaxial deposition process produces films with substantially uniform resistivities. It has also been found that uniformity of temperature aids in control of film thickness.

In accordance with the present invention, it has been found that by removing selected portions of the graphite heater at selected areas along its length by drilling small apertures, it is possible to reduce the cross sectional area of such portions and thereby alter the resistance characteristics of these particular areas. In this manner, it is possible to maintain a substantially non-uniform electrical resistance characteristic throughout the entire length of the heater and this, in turn, provides substantially uniform temperature distribution throughout the effective length of the heater.

The present invention also provides a modified form of graphite heater with substantially widened legs. Moreover, the legs are of the modified form of heater. The legs do not have a uniform cross sectional thickness but have a smaller cross sectional thickness at their transverse ends, that is the free ends which are adapted for ultimate attachment to a furnace. In this manner, it is possible to maintain a substantially non-uniform electrical resistance characteristic by adjusting the thickness of the bridge legs so that it is possible to maintain a substantially uniform temperature distribution for the effective length of each of the legs.

In the present invention, the effective length of the legs of the heater refers to that portion of a length of a leg which presents a usable surface for accommodation of silicon wafers.

Referring now in more detail and by reference characters to the drawings which illustrate practical embodiments of the present invention, A designates a graphite heater or so-called bridge generally comprising a pair of horizontal legs 1, 2 which are connected by a bight portion 3. The legs 1, 2 are provided with top and bottom faces 4, 5 respectively, interior faces 6 and exterior faces 7.

Each of the legs 1, 2 is drilled near the free end thereof to provide small apertures 8 for selectively reducing the cross sectional area of each of the legs 1, 2. The apertures 8 extend the width of each of the legs 1, 2, that is from the exterior face 7 to the interior face 6. Moreover, the apertures 8 are sufiiciently small, in the range of approximately inch diameter in order to eliminate any efiect on the structural characteristics of the heater A. It has been found that by selectively drilling the apertures 8 at a point midway between the upper and lower faces of each of the legs 1, 2 there is practically no reduction in internal strength. In many cases, it has been found that superior results are obtained when the apertures 8 are separated by increasingly smaller distances closer to the free end of the legs 1, 2. Thus, the spacing between each of the apertures 8 is increased as the distance of the aperture from the free end of the legs 1, 2 increases in the form of a geometric proportional increase.

' It is not necessarily preferable to space the apertures 8 in the form of a geometric progression. In some cases, depending upon the particular graphite heater, it has been found that suitable results are obtained when the apertures 8 are located in groups of variable spacings or of constant spacings. For example, in the graphite heater illustrated in FIGURE 1, it was found that a group of apertures spaced from each other by a distance X, followed by a second group of apertures spaced from each other by a distance of 2X, followed by a third group of apertures spaced from each other by a distance of 3X was found to produce very suitable results. The method of selectively placing the apertures is hereinafter described in detail.

It is also possible, but not absolutely necessary to provide a similar set of apertures 9 near the ends of the legs 1, 2 which are formed with the bight 3, substantially as shown in FIGURE 1. Again, each of the apertures 9 is placed midway between the top and bottom faces of the legs 1, 2 and is located so that it can present a substantially uniform temperature distribution across the length of the legs 11, 2. It has been found that by selectively drilling the apertures in this manner, the strength of the entire heater A is maintained, and yet the cross sectional area is adjusted so that a substantially uniform temperature distribution is maintained throughout its entire length.

It is also known that the current passing through the legs 1, 2 and through the bight portion 3 has a tendency to travel through the shortest current path in materials such as the graphite, from which the heating element A is formed. For this reason, the conventional heaters usually experience large current densities around the inner point of connection between the bight 3- and the legs 1, 2. This type of condition creates a substantially higher temperature along the inner margin of the bight portion 3 and a substantially cooler temperature at the outer corners of the common connection of the legs 1, 2 and the bight 3. In order to eliminate this condition, the heater A is formed with a pair of recesses 10 which extend toward the exterior wall of the bight 3, in the manner as shown in FIGURE 1. The recesses 10 are somewhat circular and extend for the full vertical length of the bight 3, that is, they extend from the top face thereof to the bottom face thereof. It has been found that by producing this type of recess, the current t-raveling through the legs 1, 2 is forced to move around the recess 10 and thereby create a higher temperature in a region which extends centrally along the bight 3. Moreover, the area between each of the recesses 10 forms a heat sink for heat dissipation in the region of high current density. In this manner, it is possible to provide a more substantially uniform temperature distribution throughout the length of the bight 3. Furthermore, this type of arrangement has eliminated the conditions of the prior art where the ends of the legs 1, 2 were substantially cold with respect to the inner margin thereof. In effect, the recesses 10 thereby form a heat sink 11 which is located between each of the legs 1, 2.

It is possible to provide a modified form of graphite heater B substantially as shown in FIGURES 3 and 4, generally comprising a pair of horizontal legs 12, 13 which are connected by a bight portion 14. The legs 12, 13 have top faces 15, bottom faces 15' interior faces 16 and exterior faces 17. Moreover, the legs 12, 13 are tapered from each of their transverse ends that is their free ends, which are adapted for ultimate attachment to a suitable furnace, such as an epitaxial silicon furnace. They are tapered in such a manner so that they have a slightly smaller across sectional thickness at each of the ends than in the center portion thereof, when referring to the vertical dimension of the legs 12, 13 reference being made to FIGURE 5. Thus, it can be seen that the thickness of the legs increases as the distance from the free ends thereof increases. The angle of taper of each of the legs 12, 13 is so adjusted so that a cross sectional area of the legs 12, 13 is maintained in order to provide a substantially constant uniform temperature distribution across the lengths of the legs 12, 13.

Each of the legs 12, 13 is connected by the bight 14 which is slightly thicker in the vertical dimension, reference being made to FIGURE 3, than each of the legs 12, 13. For theaters having overall lengths of approximately 22 inches, the legs have an overall thickness of approximately 0.260 inch at the center portion and a thickness of approximately 0.215 inch at each of the ends. The bight 14 has an overall thickness of approximately 0.500 inch and has an overall length of approxi mately 1% inches so that it is substantially thicker than the overall thickness of the legs 12, 13. Moreover, by reference to FIGURE 4-, it can be seen that the distance separating each of the legs 12, 13 has been sub t reduced so that it is only approximately; A3 of an inch spacing. By reducing the spacing between ieachof the legs 12, 13, it has beenfound that it is possible to eliminate much of the radiation from the internal walls of the legs .12, 13 and thereby maintain a more uniform temperature distribution across the length of each of the legs 12, 13. The graphite heater B is also provided with a circular recess 19 at the point of connection with each of the two legs 12, 13. The circular recess 19, which has a diameter of approximately V2 inch serves substantially the same purpose as each of the recesses in the graphite heater A. In addition, the thicknessof the bight 14 provides a larger current path so as to reduce'the current density and thereby aids in maintaining uniform temperature across the bight 14.

At each of the free ends of the legs 12, 13 that is the ends where they are normally attached to a clamp forming part of a furnace, the legs integrally merge into enlarged terminal portions 20, 21 respectively; The enlarged portions have a length of approximately 1 inches and have an overall thickness of approximately 4 inch. Moreover, the terminal ends 20} 21. mergewith the legs 12, 13 at an angle of taper of approximately 30. It has been found that by selectively adjustingthe angle of taper in order to adjust the thickness of the legs 12, 13, it is possible to provide a substantiallyuniformtemperature distribution across the entire length of the legs 12, 13. It should be understood that the present invention is not limited to heaters with only two legs and heaters with the particular shape and dimensions illustrated. For example, it'is possible to provide heaters in ac cordance with the present invention which have three or more legs connected by a common bight portion; In-this event, each of the legs of the heaters wouldbe compensated in cross sectional area in'order to maintain uniform temperature distribution across each of the legs. -In order to selectively alter the. effective cross sectional area of the graphite heaters at selected portions of their length, the graphite heaters are normally mounted in a suitable epitaxial silicon furnace in the same manner as when used in a commercial operation. The free ends of the heater are secured in clamps which may or may not be water cooled, depending upon the particular furnace.

It is, of course, understood that the heater is normally cleaned for use in the same manner as it would be if it were to be used in a commercial operation.

Thereafter, it is necessary to measure the temperature produced at various selected portions along the length and width of the heaters. This can be accomplished by attaching thermocouples to the heater and connecting the leads thereof to a suitable temperature readout device. However in actual practice, it has been found to be convenient to use an optical pyrometer. The graphite heater is then enclosed in a bell jar and current is passed through the heater in order to raise the temperature to a point within a range of normal operating temperatures. After the temperatures along selected portions of the length of the heater are recorded, the heater is cooled and. the bell jar or so-called hat is removed. At this point, the desired cross sectional area of thelegs of the heater can be determined by simple electrical relationships. After the desired cross sectional area at selected portions along the the length have been determined for the entire length thereof, the desired cross sectional area can be attained by removing the required amount of the cross sectional area. As pointed out above, this is accomplished by drilling small apertures. The apertures are sufficiently small so that they do not interfere with the internal strength of the heater, but yet, are sufiicient in number so that they sufficiently alter the cross sectional area of the legs to accomplish the intended purpose. 1

In the case of the heater B, the variance in temperature for the new heater can be determined in the manner described above in connection with the heater A. Thereafter, it is possible to shave the upper and lower surfaces of the heater- B in order to achieve the proper amount of taper so that a proper cross sectional area is maintained atselected portions along the length of the heater. The invention is further illustrated by but not limited to the following examples:

Example 1 overall length of approximately 24 inches and an overall by approximately 1 4 inches. The bell jar temperature width of approximately 3%; inches was mounted in an epitaxial silicon furnace. Each of the arms had a width of approximately 1%. inches in the transverse dimension. Moreover, each of the legs had an overall thickness of approximately A inch and each of the legs was separated was maintained at approximately 730 C. throughout the entire experiment. The jar was maintained under the pressure of 40 microns, and the power supplied to the heater was 28 volts at 470amperes. V

' An optical pyrometer Wasused to measure the temperature of each leg at each of the twenty-three wafer positions on each leg. Hydrogen gas was then passed into the bell jar at a rate of 14 liters per minute at room temperature and the following temperatures of each of the wafer positions was measured and are recorded below.

TABLE I Water Position Distance From Temperature,

N 0. Attached Tenninal (3,

' (inches) 70 The wafers nearest the free terminals of the heater are URE 5 has two curves inasmuch as each of the legs of the graphite heater was not of the same temperature across its length.

Example 2 This example also illustrates the non-linear temperature distribution across the length of the heater, similar to the heater in Example 1 when the heater is not compensated by alteration of the cross sectional area at selected portions of its length. Hence, the heater in this example was also not capable of maintaining a relatively constant uniform temperature distribution across its entire length. The graphite heater used in this example has approximately the same dimensions as the graphite heater employed in Example 1. However, the bell jar temperature was maintained at 730 C. at the start of the experiment and at approximately 655 C. at the end of the experiment. The bell jar was maintained under a pressure of approximately 40 microns and this power supplied to the heater was 26 volts at 435 amperes.

An optical pyrometer was used to measure the temperature of twenty-two wafer positions on one leg of the heater and twenty-one wafer positions on the opposite leg of the heater. The bell jar was maintained under a vacuum of 40 microns and the following temperatures of each of the wafer positions were measured and are recorded below.

TABLE II Wafer Position Distance From Temperature,

No. Attached Terminal 0.

(inches) Left Leg: 1.

3 1, s 4 1, 09a 5 1, 122 6 1, 125 7 1, 138 s 1, 144 9 1, 150 10 1, 157 11 1, 160 12 1, 163 1a 1, 163 14 1, 164 15 1, 164 16 1, 160 17 1, 161 18 1, 161 19 1, 15s 26 1, 15s 21 1, 155 22 1, 150 23 1, 100 J Example 3 This example illustrates a temperature distribution across the legs of the graphite heater when the latter has been compensated for cross sectional area in order to maintain uniform temperature distribution. A graphite heater having an overall length of 24 inches was suitably mounted in an epitaxial silicon furnace and enclosed by a bell jar. The heater had an overall width of approximately 3% inches and each leg had an overall width in the transverse dimension of approximately 1% inches. The two legs had reliefs which were formed by slots having a thickness of inch and extended inwardly from the interior surface of the bight portion for a distance of approximately inch. The slots terminated in slightly enlarged apertures having a radius of approximately inch. Each of the legs is provided with 17 apertures extending for the entire width of the leg and having a diametral cross section of approximately 0.067 inch. The 17 apertures started at a point 3 inches from the free terminal margins of each of the legs and extended to a point 5 inches from the free terminal margins of the legs or for an overall distance of 2 inches. Accordingly, the 17 holes were spaced by distances of /s inch. The legs were then provided with 8 apertures extending to a length of 7 inches from the free terminal margins and accordingly, each of the 8 apertures was separated by distances of A inch from each other. Thereafter, each of the legs Was provided with two apertures separated by /2 inch and two additional apertures separated by 1 inch thereby providing a total of 29 apertures which extended from a length 3 inches inwardly from the terminal margins of the legs to a point 10 inches inwardly from the free terminal margins of the legs.

The heater was maintained under a vacuum of 20 microns at room temperature at the outset and then hydrogen was passed into the hat at a rate of 15 liters per minute at atmospheric pressure. Current was supplied to the heater until the hat temperature was raised to 740 to 750 C. throughout the experiment. The power supplied to the heater was at 30 volts with 455 amperes.

An optical pyrometer was used to measure the temperature of 21 silicon wafer positions on each of the legs and the temperatures and the distances from the free terminal margins of the legs are recorded in the following table. (See Table III below.) The temperature of the bridge was measured and tabulated in Table III as a function of the bridge length for three different power settings. These data of temperature versus bridge length for each of the power settings were plotted as shown in FIGURE 6.

TABLE III Distance Temperature, Temperature, Temperature, Wafer From At- 0. 0. 0. Position No. taehed Terminal (inches) (21) (b) (c) Left Leg:

By reference thereto it can be seen that the three power settings were maintained at (a) 33. volts and 465 amperes Example 4 This example also illustrates the temperature distribution across the legs of a graphite heater when the latter has been compensated for cross'sectional area in order to maintain uniform temperature distribution, in the same manner as the graphite heater of Example 3. The graphite heater employed in this particular example was substantially similar to the graphite heater employed in Example 3 and was similarly secured to an epitaxial silicon furnace and enclosed by a bell jar. The apertures formed in the side walls of the legs were substantially the same as those formed in the heater of Example 3.

The heater was maintained under a vacuum of approximately 20 microns at room temperature. The heater was then vacuum purged, purged at room temperature and then hydrogen was passed into the hatv ata rate of 15 liters per minute. Current was supplied to the heater until the hat temperature was raised toapproximately 740 C. to 750 C. throughout the experiment. The power supplied to the heater was 30 volts at 455' amperes. After approximately fifteen minutes the temperature of the Recorded in the column designated as Temperature No. 2" is the temperature of the bridge after the addition of the trichlorosilane.

These temperature distributions could also be plotted as a function of the length of each of the legs to obtain curves similar to those plotted in FIGURE 7. It could thus be seen that asubstantially uniform temperature This example illustrates the temperatureldistribution across the legs of the graphite heater B which has been compensated for crosssection-al area in order to maintain uniform temperature distribution; The graphite heater B had an overall length of 24 inches and an overall width of 5% inches. Each leg had a width of approximately 2% inches separated by a slot of 4; inch between each leg. Each of the legs of the graphite heater had an overall thickness in the vertical dimension of approximately Mt inch. At its free end, the graphite heater ter- 'trichlorosilane was passed into the hat at a rate of approximately 0.5 gram per minute. I

A total of 19 silicon wafers was placed upon each of the legs and the distances from the free terminal margins of the legs are recorded inthe table as set forth below.

TABLE IV Distance From Attached Ter- Temperature o. 1, C. minal (inches) Temperature Wafer No. No. 2, C.

minated in an enlarged head portion having a length of 1 4 inches-and an overall thickness of .260 inch, at a point midway between their length. At the bridge end,

each of the legs was integrally formed withenlarged head 7 portions having a length of approximately 1% inches legs had overall thicknesses of approximately 0.215 inch. At the head portion, the enlarged bridge was formed with an aperture having a diameter of approximately /2 inch, and is located at the point midway between eachof the legs.

The heater was attached to an epitaxial silicon furnace having means for water cooling the heater at the point of attachment and was enclosed within a bell jar. The cooling water inlet temperature was maintained at 27 C. and the outlet water temperature was recorded to be 62 C. At the start of the operation, the pressure within the bell jar was reduced to 200 microns. The bell jar was then purged with gaseous hydrogen for approximately 10 minutes. Gaseous hydrogen was thereafter passed into the bell jar at approximately 35 litersper minute (standard pressure and temperature). Current was then supplied to the heater until the temperature of the bell jar was raised to 780 C. The power supplied to the heater was 17.2 volts at 1070'amperes. v 1

Shortly thereafter trichlorosilane gas was passed into the heater at a rate of 1.28 grams per. minute. A doping gas, phosphorus triehloride, was added to the bell jar at a rate of approximately 25 milliliters per minute. A total of 35 Wafers were placed on each leg of the heater and temperatures are recorded'in the table set forth belo'w' (Table V). It is to be noted that there were two rows of waters on each' leg. The temperature of the heater was plotted as a function of the heater length as illustrated in FIGURES 7, and 8 and shows the relatively uniform temperature distribution across the length of the legs.

The graph of FIGURE 7 shows the temperature of the wafer positions in each row on the left leg, and the graph of FIGURE 8 shows the temperature of the wafer positions in each row on the right leg.

TABLE V Distance Distance From Temperature, From Temperature, Wafer No. Attached 0., Water No., Attached 0., Outer Row Terminal Outer Row Inner Row Terminal Inner Row (inches), (inches). Outer Row Inner Row It should be understood that changes and modifica- 3. The electrical heating element of claim 1 further tions in the form, construction, arrangement and cornbicharacterized in that said recess in substantially larger nation of parts presently described and pointed out may than the spacing between each of said legs. be made and substituted for those herein shown without 4. The electrical heating element of claim 1 further departing from the nature .and principle of my invention. characterized in that said heating element has only two Having thus described my invention, what I desire to legs. claim and secure by Letters Patent is: 5. The electrical heating element of claim 1 further 1. An electrical heating element formed substantially characterized in that an enlarged relatively thick terminal of carbonaceous resistance material for use in epitaxial portion is for-med at the free end of each of said legs, said deposition furnaces and the like and being adapted to relatively thick terminal portion being substantially be heated by the passage of an electric current therethicker in the vertical dimension than the point centrally through; said heating element comprising a plurallty of of the length of the legs. longitudinally extending substantially coplanar legs, a bight portion extending across and operatively connect- References Cited ing one of the transverse ends of each of said legs, said UNITED STATES PATENTS legs having free opposed transverse ends adapted for operative attachment to a source of electrical power, gnderson 338-138 X each of said legs being longitudinally tapered from points 17 5/1 45 lmpson 338-293 X centrally of their length to each of the transverse ends so 2569773 1 3 gloom et a that the cross sectional area of each leg is reduced as the 2615060 18/1952 219 543 distance from the center of the legs to the transverse 9 1 54 g eta 13-25 ends thereof is increased, said taper being sized to pro- 3 5 c erman 338' 217 X duce differences in the longitudinal cross sectional area 1 9 4 AFxander 1325 and in resistance characteristics of each of said legs of 2 811961 A i' 219 553 X id heating element and creates a substantially uniform 37160593 12 1964 Pa 13-22 temperature across the length of said legs, said heating 3,299,398 1/1967 Vercesl et a1 338 217 X element having a recess formed in said big-ht portion FOREIGN PATENTS :ad acenttsmd legs at the pomtpf vatfta ghmenlt torsard bighctit 5 71,798 7/1959 France portion '0 mam am un1 ormi y o empe an e 1n ear 6 361,960 11/1931 Great Britain.

bight portion.

2. The electrical heating element of claim 1 further characterized in that said bight portion is substantially thicker in cross-sectional area than any of said legs.-

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

P0405 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,3S1 742 Dated November 7, 1967 Inventor) D. M. Harris It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

F- Column 4, line 53, delete "across" and insert "cross".

Column 8 Table III, third column, the tenth number from the bottom should be "I, 187".

Column 10, line 37, delete "At the head portion, the enlarged bridge" and insert --At the bridge portion, the enlarged head--.

Claim 5 should be dependent upon claim 2 and not claim 1.

SIGNED AND SEALED (SEAL) Attest:

Edward M Fletch", WILLIAM E. 50mm, 311. Investing Offi Commissioner of Patents 

1. AN ELECTRICAL HEATING ELEMENT FORMED SUBSTANTIALLY OF CARBONACEOUS RESISTANCE MATERIAL FOR USE IN EPITAXIAL DEPOSITION FURNACES AND THE LIKE AND BEING ADATPED TO BE HEATED BY THE PASSAGE OF AN ELECTRIC CURRENT THERETHROIUGH; SAID HEATING ELEMENT COMPRISING A PLURALITY OF LONGITUDINALLY EXTENDING SUBSTANTIALLY COPLANAR LEGS, A BIGHT PORTION EXTENDING ACROSS AND OPERATIVELY CONNECING ONE OF THE TRANSVERSE ENDS OF EACH OF SAID LEGS, SAID LEGS HAVING FREE OPPOSED TRANSVERSE ENDS ADATPED FOR OPERATIVE ATTACHMENT TO A SOURCE OF ELECTRICAL POWER, EACH OF SAID LEGS BEING LONGITUDINASLLY TAPERED FROM POINTS CENTRALLY OF THEIR LENGTH TO EACH OF THE TRANSVERSE ENDS SO THAT THE CROSS SECTIONAL AREA OF EACH LEG IS REDUCED AS THE DISTANCE FROM THE CENTER OF THE LEGS TO THE TRANSVERSE ENDS THEREOF IS INCREASED, SAID TAPER BEING SIZED TO PRODUCE DIFFERENCES IN THE LONGITUDINAL CROSS SECTIONAL AREA AND IN RESISTANCE CHARACTERISTICS OF EACH OF SAID LEGS OF SAID HEATING ELEMENT AND CREATES A SUBSTANTIALLY UNIFORM TEMPERATURE ACROSS THE LENGTH OF SAID LEGS, SAID HEATING ELEMENT HAVING A RECESS FORMED IN SAID BIGHT PORTION ADJACENT SAID LEGS AT THE POINT OF ATTACHMENT TO SAID BIGHT PORITON TO MAINTAIN UNIFORMITY OF TEMPERATURE IN SAID BIGHT PORTION. 